As Persistent Marshstalker returns to the battlefield because of its triggered ability, you choose which player, planeswalker, or battle it's attacking. It doesn't have to attack the same player, planeswalker, or battle as your other Rats.

Although Persistent Marshstalker's last ability causes it to enter attacking, it was never declared as an attacking creature. Abilities that trigger whenever a creature attacks won't trigger when it enters attacking.